If the sacrificial animal becomes damaged after selecting it

Firstly: 

If a person has selected a sacrificial animal, then some
damage befalls it that is not caused by any wrongdoing or negligence, then
he slaughters it at the time of slaughtering, then it is valid and is
counted as a sacrifice. 

Ibn Qudaamah (may Allaah have mercy on him) said in
al-Mughni (13/373): 

If the sacrificial animal is sound and free of faults, then
something happens to it that renders it unsuitable, it should be slaughtered
and it still counts as a sacrifice. This was narrated from ‘Ata’, al-Hasan,
al-Nakha’i, al-Zuhri, al-Thawri, Maalik, al-Shaafa’i and Ishaaq. End quote. 

The evidence for that is the report narrated by al-Bayhaqi
from Ibn al-Zubayr (may Allaah be pleased with him), according to which a
one-eyed she-camel was brought to him as a sacrificial animal, and he said:
If this happened to it after you bought it, then go ahead (and sacrifice
it), but if it happened to it before you bought it, then offer another one
instead of it.” Al-Nawawi said in al-Majmoo’ (8/328): its isnaad is
saheeh. 

Shaykh Ibn ‘Uthaymeen said in his essay Ahkaam al-Udhiyah
(Rulings on the Sacrifice) concerning the rulings on selection of the
sacrificial animal: 

If the animal has a fault that renders it unsuitable for
sacrifice, one of two scenarios must apply: 

1 – That was caused by the owner’s wrongdoing or negligence,
in which case it must be replaced with another animal that is the same or
more perfect, because if the damage was caused by him, he must replace it
with another one to be slaughtered instead, and the damaged animal still
belongs to him, according to the sound scholarly opinion; he may do whatever
he wants with it, sell it or whatever. 

2 – The fault was caused without any wrongdoing or negligence
on the owner’s part. In that case he should sacrifice it and it counts as a
sacrifice, because it is a trust that is in his keeping but it got damaged
through no action or negligence on his part, so there is no sin on him and
he does not have to replace it. End quote. 

Secondly: 

How is a sacrificial animal selected? 

The selection of the sacrificial animal may be verbal, such
as saying, “This is a sacrificial animal.” 

If they buy it with the
intention of offering it as a sacrifice, Abu Haneefah and Maalik (may Allaah
have mercy on them) are of the view that the selection has been made
thereby. Al-Shaafa’i and Ahmad are of the view that the selection is not
made thereby. 

The scholars of the Standing Committee are of the view that
the selection is made when one buys the animal with the intention of
offering it as a sacrifice. 

It says in Fataawa al-Lajnah al-Daa’imah, 11/402: 

The sacrificial animal is selected when one buys it with the
intention of offering it as a sacrifice or when one selects it. End quote. 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
said in Majmoo’ al-Fataawa (26/304): 

If a person
buys a sacrificial animal and it gets damaged before slaughter, it may be
slaughtered according to one of the two scholarly opinions. End quote. 

Based on the above, if you bought this sheep with the
intention of offering a sacrifice, then it gets damaged without any
wrongdoing or negligence on your part, then it is counted as a sacrifice, in
sha Allaah. 

And Allaah knows best.